1. Field of the Invention
The present invention relates to a vehicular power transmission device which utilizes the viscous resistance of a fluid.
2. Description of the Related Art
In an automobile with an all-wheel drive system of the type in which the wheels on the front axle are usually driven, an automatic power transmission device is known as shown in Japanese Publication of Unexamined Utility Model Application No. 59-188731 which relies on the fact that the front wheel axle and the rear wheel axle are usually mutually connected in an almost rigid manner based on the torque with a viscous clutch which is positioned between the front wheel axle and the rear wheel axle in the drive system thereby driving the wheels on the rear axle.
Where there is a slight difference in RPM between the input shaft and the output shaft, the viscous clutch transmits only an extremely small torque. In contrast, when there is a large difference in RPM, the viscous clutch has the function of transmitting an extremely large torque.
In such a conventional power transmission device, a flange is provided on one end of an input shaft. A bearing hole is formed on the centerline section of the input shaft in the axial direction to support the tip of an output shaft. One side of a cylindrical outer tube engages the outer periphery of the flange and after engaging, the two are integrally connected at the external peripheral ends by a welded section.
A plurality of channels (for example, channels for receiving a spline) are provided parallel to the centerline on the inner circumferential surface of the cylindrical outer tube. A plurality of teeth (for example, teeth for receiving a spline) equivalent in number to the number of channels are provided on the outer peripheries of a plurality of input shaft clutch plates, which act as resistance plates inserted into the channels. These teeth are slidably fitted in the channels which are provided on the outer tube. A hollow portion with a slightly large diameter is formed in the center section of the input shaft clutch plate in such a way that it does not contact the outer periphery of an output shaft.
The input shaft clutch plates are mutually assembled with a plurality of output shaft clutch plates with a prescribed gap therebetween. The assembly maintains the gap as set by a stopper and a maintaining plate which is provided at the distal edge of the assembly.
A stopper for preventing the separation of the outer tube of the input shaft from the output shaft, is mounted as a ring on the tip of the outer tube.
In such a conventional power transmission device as mentioned above, the input shaft clutch plate, which is provided on the input shaft for the transmission of power, transmits its rotational power to the output shaft clutch plate through the highly viscous fluid which is sealed into the section chamber to provide for the production of rotation in the output shaft, wherein the phenomenon of tight corner braking at low speeds of the output shaft can be prevented from occurring. However, because the power is transmitted through a viscous fluid, slipping is always produced between the clutch plates and the viscous fluid, and the viscous fluid heats up because of this slipping, lowering the viscosity of the fluid. This in turn lowers the transmission limit torque.
As a result, a waste of power transmission is produced, which prevents the automobile from running smoothly.
In addition, as a conventional power transmission device of this type, there is also a device as described in the Japanese Patent Publication of Examined Application No. 58-48779.
In this device, when there is a difference in the rotation between the front and rear wheels, a resistance plate rotates in a viscous fluid filled into a sealed action chamber, utilizing the viscous resistance to transmit the power. However, when the rotational differential continues, the temperature of the action chamber rises, causing its pressure to rise, so that a piston activated by this pressure moves in the axial direction, causing an outer clutch mechanism to engage so that the differential is removed and the temperature raise is restrained.
However, with such a conventional device, the durability of the seal member is reduced because the differential is removed by the movement of the piston resulting from the rise in pressure in the differential chamber to prevent a rise in temperature. This is because the internal pressure itself must be maintained at a substantially higher level to obtain engaging or clamping power by means of the internal pressure rise. In addition, when the differential is removed with the internal pressure dropped by lowering the temperature, the engagement of the clutch mechanism is released and once again the rotational differential occurs. The internal pressure once again rises from this rotational differential and removal of the differential is carried out, causing the action to repeat in this way, so that this action is unstable.